Changes in background cosmic radiation dose during the history of life on earth

All living organisms are exposed to a constant barrage of DNA damage from a variety of sources, including radiation. To maintain genomic fidelity it is important to repair this damage, and virtually all living organisms have DNA damage repair mechanisms. Many of these repair mechanisms probably date to the earliest living organisms, so understanding the mutagenic environment in which life originated may lead to a more complete understanding modern organisms to DNA damage response.; Background ionizing radiation is one of the sources of DNA damage, and all sources of background radiation have changed with time. Other factors, most importantly the presence of free oxygen in the atmosphere, can strongly modify the amount of damage done by radiation. In this dissertation, I examine the manner in which several sources of radiation have changed through time and how changes in atmospheric oxygen concentrations have affected genetic damage from exposure to background radiation.; Solar evolution models were used to examine the manner in which cosmic radiation levels have changed with time. In conjunction with atmospheric chemistry models (which describe changes in ozone concentrations), it is apparent that sea level UV flux has dropped by a factor of over 400 since life's first appearance while radiation dose from cosmic rays is now slightly less than one half the levels that existed four billion years ago.; Oxygen is also important as an enhancer of radiation damage to DNA. This effect is examined using models for the evolution of atmospheric oxygen concentrations and suggests that early high levels of radiation were somewhat countered by low levels of, oxygen, so the earliest life was probably exposed to only 2–4 times current levels of radiogenic DNA damage.; Finally, it appears as though gamma radiation from supernovae, gamma ray bursts, and supernovae-produced radioactivities provide infrequent and aperiodic bursts of gamma radiation that can produce relatively high (although not lethal) radiation levels at sea level. However, these events may be more important as constraints on the ability of living organisms to travel between planets.